Slow burning propellent grain with polysulfide polymer coating



Aug. 1, 1961 w. A. GEY 2,994,597

SLOW BURNING PROPELLENT GRAIN WITH POLYSULFIDE POLYMER comma Filed Feb. 20, 1953 co 00 no 500 PSI 500 PSI I000 PSI I500 PSI INVENTOR.

WILLIAM A. GEY

*tatcs 2,994,597 SLOW BURNING PROPELLENT GRAIN WITH POLYSULFIDE POLYMER COATING William A. Gey, China Lake, 'C-alif., assignor to the United States of America as represented by the Secretary of the Navy Filed Feb. 20, 1953, Ser. No. 338,177 1 Claim. (Cl. 52-.5) (Granted under Title 35, US. Code (1952), see. 266) The inventiondescribed herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to slow burning propellent grain and the method of making it.

In most applications of propellents, such as rocketry, a propellent grain having a fast burning rate is desired. However, in some applications, such as the driving of servo-mechanisms and other similar devices, a slow burning propellent grain which gives a longer operation time for the device powered by it is required. Propellent grains ordinarily used to power high speed devices have too rapid a burning rate for use in servo-mechanisms. In addition, previous propellent grains are subject to the disadvantage that separations are formed in them under operating conditions because of the widely different coefiicient of expansion of the compressed grain and the adjacent container wall. This results in irregular burning which causes excessive and widely varying pressures which are impossible to control, thus making the use of previous propellent grains highly impractical in such devices as servo-mechanisms which must be susceptible of precise control.

It is, therefore, an object of this invention to provide a slow burning propellent grain.

It is another object of this invention to provide a propellent grain for use in servo-mechanisms which can be packaged and used under operating conditions without the formation of separations due to different coefficients of expansion of the compressed grain and the adjacent container wall.

It has been found that the above objects can be accomplished and the enumerated difficulties overcome by a propellent grain coated with a restrictive coating of a cured polysulfide polymer.

The propellent used to illustrate this invention comprises a mixture of about 75 parts of nitroguanidine, 25 parts of guanidine nitrate, and 8 parts of ammonium dichromate. The nitroguanidine used is a high bulk density product. The guanidine nitrate is practical grade, ground to pass a 100 mesh screen. The ammonium dichromate is chemically pure, or practical and is ground to pass a ZOO-mesh screen.

The propellent grain in this case was formed by compressing the dry powder mixture in a 1.125" diameter polished steel mold at 27,000 pounds per square inch. In order to properly compact the powder, two pressings of 23 grams each are made. The ram for the first pressing contains a conical cavity in order to insure proper adhesion of the second increment. The ram for the second pressing is flat. After pressing, the burning surface was modified by cutting cross slots three thirty seconds inch wide into the face with a circular saw to a depth of nine-sixteenths inch. This cavity is required to obtain the desired burning area to nozzle area ratio of 1180 which yields an operating pressure of 1000 psi. at ordinary temperatures. The larger initial burning area also compensates for the initial heat loss to the cold metal chamber.

The function of the restrictive coating or liner is to absorb the differential thermal expansion between grain and container and maintain a tight fit against the gain. It must be strong and stifi with a surface hard and smooth enough to resist damage or slippage during loading of the fuel. Combined with these properties the coating of this invention has the property of restricting the burning surface of the propellent grain. The restrictive coating used is a liquid polysulfide polymer or poly-functional mercaptan, designated commercially as Thiokol, LP-2, which has been cured. Thiokol LP-2 is made in accordancewith the process set forth in US. Patent 2,466,963, granted April 12, 1949, to Joseph C. Patrick and Harry R. Ferguson. The polymer has the following properties:

Properties Molecular weight, av 4000. Viscosity, poises 350 to 450. pH 6-8. Specific gravity 1.27. Color Amber.

Stability Indefinite. Moisture content Less than .2%.

SH terminals Extremely reactive.

The burning surface restrictant, Thiokol LP2, was cured by mixing parts of it with a previously mixed suspension of 9 parts of lead oxide, 8.5 parts of dibutyl phthalate, and 0.5 part of stearic acid, and warming at 50 C. for 16 hours. Chemically pure lead oxide, PbO must be used, as the mixture is sensitive to impurities and will cure rapidly in the cold in their presence. The procedure for application of the restrictant consisted in painting the grain (except for the coned end) with a cold-curing, rubber-to-metal cement, for example, that designated by Navy specification No. 52-0-15 Type I, drying un-til tacky, placing it in a mold which had been treated with silicone release agent, and pouring the uncured mixture of inhibitor around the grain. The curing operation was carried out under 4 to 5 inches of vacuum to remove air bubbles. A firm rubbery coating .060 inch thick was obtained. The inhibitor was found to control the burning rate of the grain, the finished grain of a diameter of 1% inches burning at a rate of .14 inch per second at 1000 psi. This burning rate is in contrast to that of prior propellants which have burning rates impossible to check. When tested in a servomotor the grain proved satisfactory.

The rubbery quality of the inhibitor coating removes the disadvantages formerly resulting from different coefficients of expansion of container and grain, i.e., separations of the grain. The inhibitor coating forms a moisture-proof enclosure for the grain and enhances both its storage qualities and handling characteristics.

The drawing shows a typical pressure-time trace of the propellent grain illustrating its burning characteristics.

While there has been disclosed a preferred embodiment of my invention in which a specific propellent composition is used with the cured polysulfide restrictant, to illustrate the invention, the example is by no means intended to be limiting. It will be apparent to those skilled in the art that the weight proportions of the components in the propellant composition may be varied depending upon the burning rate required. Also, propellants of totally different composition may be used in combination with the burning surface restrictant.

It is thus seen that there has been disclosed a propel-lent grain adapted for use in servo-mechanisms and other mechanisms of this type which has a slow burning rate,

Patented Aug. 1, 1961 good handling and storage characteristics and which is easily fabricated.

Obviously many modifications and variations of the present invention are possible in the light of the above teaching. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

A propellent grain consisting essentially of about 75 parts by weight of nitroguanidine, about 25 parts by weight of guanidine nitrate, about 8 parts by weight of ammonium dichromate, and a coating on the outer surface of said grain consisting of a cured polysulfide polymer having the average structure 4 References Cited in the file of this patent UNITED STATES PATENTS 2,195,380 Patrick Mar. 26, 1940 2,479,828 Geckler Aug. 23, 1949 2,555,333 Grand et a1. June 5, 1951 2,584,264 Foulks Feb. 5, 1952 2,637,274 Taylor et a1. May 5, 1953 2,643,184 Cairns et al June 23, 1953 OTHER REFERENCES 

